Subtension
What a given division on the reticle (one mil, one MOA, one click) actually covers at a given distance. The angular value of every reticle feature.
A 1-mil hash mark subtends 3.6 inches at 100 yards, 36 inches at 1,000. A 1/4-MOA click subtends roughly 0.26 inches at 100 yards. Knowing these values lets you read distance, size targets, and call corrections fluently.
Manufacturers publish subtension diagrams for every reticle they offer. Print one and laminate it; the reticle becomes a measuring stick for everything in the field of view.